Traditional wireless communication systems use a single transmission path containing an amplifier and an antenna. Such systems are known as Single Input Single Output (SISO) systems. Multiple Input Multiple Output (MIMO) systems have link budget capacity and throughput superior to SISO systems. MIMO systems deliver greater performance, but with additional cost and power consumption. Thermal and battery life constraints limit tolerable power consumption in wireless portable devices. Moreover, the wireless standards limit the output power to certain levels. The requirement to support traditional SISO systems even aggravates the situation. In SISO mode, the transmitter should deliver the maximum power through one path. At the various MIMO modes (usually 2-4 transmitting antennas), each path should transmit less power; such that the overall power stays the same. Current consumption is expected not to change because total transmitting power stays the same, but generally, this is not practical. An amplifier designed to work at full power in SISO mode, when working at 3-5 dB less power in MIMO modes, leads to 30-55% increase in current consumption of the power amplifiers in the MIMO modes.
A conventional RF amplifier achieves its maximum efficiency at saturation level. Back off from that level reduces efficiency, but is usually required in order to meet output signal linearity conditions. In many wireless applications, several output power levels are required from the same amplifier. The power stage has to be designed to handle the highest power leading to very low efficiency at lower output power.
The technique of eXcess eNvelope eNhancement (XNN®) for power amplifiers (PA) is a solution for efficiency enhancement and power boost of power amplifiers, particularly for the low power WiFi and high power and low power WiMAX market applications, as well as for existing high power cellular applications and forthcoming cellular standards. Such solution is disclosed, for example in U.S. Pat. No. 6,831,519, in U.S. Pat. No. 6,437,641 and in US Patent Application No. 2004/0018821.
The Voltage Enhancement Circuit (VEC™) modulates the supply voltage of power amplifiers as part of the XNN® technology, disclosed in U.S. Pat. No. 6,437,641. Appropriate modulation of the supply voltage prevents saturation of power amplifiers while amplifying signals that exceed a pre-defined programmable threshold.
A MIMO system requires using of several power amplifiers, one for each transmitting path of the information signal. Conventional implementation of XNN® technology for this case requires using a VEC™ circuit for each replica of the signal. However, using several VEC™ circuits on one board might be expensive and area as well as power consuming. Therefore, a novel technique for reducing the number of VEC™ circuits required for the operation of a MIMO system is needed.
All the methods described above have not yet provided satisfactory solutions to the problem of efficiently providing power enhancement to several power amplifiers, arranged in a MIMO system.
It is an object of the present invention to provide a method for improving efficiency of MIMO amplifier arrays that supports also operating in a SISO mode.
It is another object of the present invention to provide a method for efficiently controlling the current drawn by power amplifiers, arranged in MIMO arrays.
It is another object of the present invention to provide circuitry for efficiently controlling MIMO transmitter current consumption, gain and output power independently.
Other objects and advantages of the invention will become apparent as the description proceeds.